The present invention is a composite material and method of making the same. More specifically, the present invention relies upon the use of a self assembling, fully dense monolayer as a chemical bond between a first material and a polymer.
Nearly all materials may be considered composite materials because most materials are made up of dissimilar elements, even to the molecular level. However, as used herein, a xe2x80x9ccomposite materialxe2x80x9d is a combination of dissimilar solid bulk materials, specifically a combination of a material having an oxygenated surface, a fully dense monolayer, and a polymer.
Oxygenated includes oxide, hydroxide, hydrated oxide and combinations thereof.
The market for composite material totaled nearly 1.2 million tons in 1991. It is estimated to reach 3.7 million tons by the year 2000. The use of composite material is driven by the desire to decrease part weight and/or increase the factor of safety in a design. Helmets, lightweight structural elements for vehicles, and many other products benefit from the use of composite materials. The continuing challenge for designers of composite materials is to maintain structural strength, increase dynamic strength, and decrease weight.
In many applications, the desired composite material is a polymer combined with a second material. The polymer may be in the form of a slab or sheet that is combined with another slab or sheet of the second material. The slabs or sheets may include solid or monolithic slabs or sheets, as well as woven or networked structures. Another common form is a polymer mixed with a filler wherein the filler is the second material. This form of composite material may be described as the filler having a plurality of particles with the polymer substantially surrounding each of the plurality of particles, and the polymer is bonded or interfacially adhered to each of the plurality of particles. Particles are available in various geometric forms including particulate, fiber, rod, and combinations thereof. In either case, the bond between the polymer and the second material is characterized as a mechanical strength limitation of the composite material.
Composite materials of this type are generally made by contacting the polymer with the second material so that the polymer bonds to the second material. Most often, the second material is a filler of a plurality of particles substantially surrounded by the polymer.
It is well known that these types of composite materials fail well below their theoretical strength. Failure mechanisms of composite materials are complex, but delamination, or separation of composite elements, is a major cause of structural failure of these types of composite materials. Delamination may be characterized by incomplete wetting of the filler (usually a fiber) by the polymer resin. In addition, even in well-wetted areas, there can be poor interfacial adhesion between the filler and the polymer resin. The poor bond between filler and polymer or poor interfacial adhesion is characterized as a mechanical strength limitation of the composite materials. For example, double paned windows lose the seal between the panes as a result of loss of bond integrity between the glass pane and the plastic frame structure. Fiberglass loses strength when the glass fibers separate from the polymer.
Efforts to overcome this mechanical strength limitation have resulted in placing a bond coating on the second material. Such bond coatings are usually a layer of a low molecular weight organic polymer, or organosilane coupling agents, for example phenol-formaldehyde resins or 3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, or -aminopropyltriethoxysilane. However, even with these bond coatings, the strength of the interfacial bond is far less than theoretical strength.
U.S. Pat. No. 5,759,708 to Tarasevich and Rieke shows a composite material of an organic substrate that is modified (surface modified by addition of acidic, basic or neutral group) by addition of a functional group, followed by solution deposition of a second material of an inorganic material that can be a ceramic material. A disadvantage of this method is that bulk forms (e.g. slab or particle) of both substrate and second material cannot be bonded together. In other words, the method of Tarasevich and Rieke cannot be applied to making a composite material of a filler in a polymer.
Thus, there remains a need for a composite material that is a combination of bulk forms, and method of making such a composite material that has greater mechanical strength as a result of improved interfacial adhesion between the polymer and the other material.
The composite material and methods of making of the present invention rely upon a fully dense monolayer of molecules of at least 1.5 molecules per square nanometer. The molecules of the fully dense monolayer further have a carbon chain spacer having a first end and a second end, the first end bonded to an oxygenated surface of a first material, and the second end bonded to an organic terminal group that is in turn bonded to a polymer.
It is an object of the present invention to provide a composite material with enhanced mechanical strength.
It is another object of the present invention to provide a method of making a composite material with enhanced mechanical strength.